From DE 40 08 405 C1, a microwave PICVD process for the production of a cold light reflector on the interior side of dome shaped substrates is known, which may also consist of plastics. In order to achieve a uniform thickness of coatings, a gas showerhead adapted to the contour of the surface is suggested, among other things. The known process only generally hints at how a curved substrate could be coated uniformly. If a substrate with different surface curvature should be coated--there are more than 10,000 different examples--then this process is not useful for economic reasons because of the multitude of different gas showerheads.
FR 26 77 841 describes a microwave plasma chemical vapor deposition (CVD) reactor for the production of safety and filter coatings on optical plastics substrates, such as lenses. The substrate lies on the electrode of a 13.56 MHz generator with a bias voltage. Oxygen is stimulated by a 2.45 gigahertz microwave field, which in turn stimulates the reaction partner, which is supplied at a remote location and contains silicium. As a consequence, a coating is formed on the substrate. The back side of the substrate is protected from undesired coating by an antibody, which is adapted to the form, and as a result, this process is uneconomical with substrates with different surface curvatures because of the multitude of required antibodies. FR 26 77 841 does not disclose how to attain uniformity with this approach.
DE 41 42 877 A1 describes a CVD process and a device for the coating of semiconductor wafers with uniform thickness. Uniformity is achieved by using a gas showerhead that has several individual gas blow areas. The gas showerhead is situated opposite the substrate and covered by a closed hood, which distributes the coating gas to the substrate rotationally symmetrically. The first and second gas blower area must be on a common level to ensure that a coating of high quality and uniform density can be formed, and it is therefore unlikely that this process can be used for the uniform coating of curved surfaces. A second disadvantage of this approach is the relative slowness of the coating process. Only a low uniformity of .+-.5% can be achieved, which is insufficient for the production of antireflection coatings. No suggestion concerning the coating of curved substrates with or without use of rotation symmetry is provided, and no measures are mentioned regarding how the backside of such substrates could be protected from the coating process.
DE 39 31 713 C1 describes a process and a device for simultaneous CVD coating from all sides of curved substrates in a 13.56 Megahertz Plasma. According to DE 39 31 713 C1, no difference can be detected in the thickness of the coating from the center to the periphery using this process. However, no measurement accuracy is provided. The process only refers to the precipitation of scratch resistant layers for which requirements for uniformity are much lower than for precipitation of anti-reflection reflection layers. The adaptability of this process for the production of anti-reflection coatings is neither disclosed nor suggested by this reference.
DE 34 13 019 A1 describes a plasma CVD process for the production of a scratch resistant layer to transparent plastics elements through polymerization of hexamethyldisiloxane (HMDSO) or other organic compounds containing silicium, whereby good adhesion with superior surface hardness are achieved through a continuous transition from an organic polymer to an inorganic hard protective coating. The device includes a vacuum recipient and a drum that rotates about its axis, and the plastic parts to be coated are placed at the circumference surface. The parts to be coated pass a coating unit consisting of many single nozzles, through which reaction gases are supplied. A glow discharge is activated via high voltage from outside to the monomeric steam. The production of a scratch resistant layer is possible using this process.
Klug, Schneider and Zoller describe in SPIE Vol. 1323 Optical Thin-Films III: New Developments 1990 (Page 88ff) describes a process for depositing hard and thick (2.5 to 5 .mu.m) safety coatings to CR-39 lenses through a plasma CVD process with gases from oxygen and silicium compounds. Also described is the production of a blooming coat via high-vacuum vaporization in another coating process. It is suggested not only to produce the scratch resistant layer, but also the anti-reflection coating using the PCVD process. No solution, however, is given as to how the necessary coating uniformity can be achieved with curved substrates using the same process for both coatings. In particular, the problem of the backside coating is not addressed.
U.S. Pat. No. 4,927,704 to Reed et al. also describes a plasma CVD process for the deposit of a transparent scratch resistant layer on plastics, especially polycarbonate, using two transitory coatings, one of which includes a continuous material transition from organic to inorganic. The adjustment of the thickness of the coating occurs using the parameters of mass flow of the coating gases, substrate temperature, excitation frequency, and pressure. The desired thickness of the coating can only be adjusted to .+-.5%, and the process is therefore too inexact for the production of anti-reflection coatings.
EP 0 502 385 describes a process for the production of double sided coatings of optical tools. Coating of the tools occurs with the help of evaporation sources which are situated at a certain distance opposite the tool support. For the production of uniform coatings, the tool support with the substrates is rotated during the coating process. The rotation of the tool support guarantees that the plasma discharge, which occurs before and after evaporation and which is formed between the electrodes and the tool support, which acts as a counter-electrode, covers the tools in a uniform manner. No detailed description is provided regarding the quality of the coatings or their thickness. One disadvantage of the described process is that a part of the electrode is also coated during the exposure of the evaporation source. A portion of the coating material sprayed on the electrode will therefore be lost in the tool support. The plasma CVD process can obviously only be used in the production of scratch proof layers, as an evaporation process must be utilized for the deposit of the abrasion-resistant (AR) coating, and this process cannot coat both surfaces simultaneously. In addition, this approach is not economical because two coating processes are used and because the high vacuum process is especially uneconomical.
EP 0 550 058 A2 describes a gas zone showerhead for the even coating of tools, such as semiconductor wafers. The individual zones of the gas showerhead can be deposited with different mass flows and different gases. No detailed description is provided concerning curved substrates.
U.S. Pat. No. 4,991,542 to Kohmura et al. describes a process for the simultaneous coating of both sides of even substrates using a plasma CVD process. The substrate to be coated is situated in a reaction chamber. A gas showerhead is provided opposite the sides to be coated, and the showerhead occurs via use of feed-gas supplying electrodes. High frequency voltage is applied, so that a plasma is produced between the substrate and the gas showerhead. Coating of curved substrates is not described.
In M. Heming et al., "Plasma Impulse Chemical Vapor Deposition--a Novel Technique for Optical Coatings," in 5. Meeting Opt. Interference Coatings, Tuscon 1992, the use of a gas showerhead and PICVD for the uniform coating of even substrates is described. If curved substrates are coated using this method but without fitting a counter-surface (gas showerhead) against the substrate curvature, a coating that becomes more uneven the greater the curvature of the substrate can be expected.
During the coating of substrates with a PICVD process, a gas chamber above the substrate is filled with coat forming material, which is transformed to coating material in a plasma impulse. The period of the consequent impulse pause is selected to ensure a complete exchange through unburned gas.
In the application of the PICVD process, it is assumed that a uniform coating of substrates can be achieved when the products K.sub.i .times.V.sub.i are approximately constant, or more exactly, when the integral in the first approximation is: EQU h.sub.i =h.sub.L EQU F.sub.i *.intg.K(h.sub.i)dh.sub.i .apprxeq.const EQU h.sub.i =0
whereby K(h.sub.i) is the concentration of the coating formation gas at a distance h from the substrate, F.sub.i is the surface element of the substrate, and h.sub.i the amount of gas volume that contributes to the coating formation on the substrate since the thickness of the coating obviously depends to the greatest extent on the amount of coating formation material in volume element V.sub.i above the surface element to be coated.
A numerical simulation of the flow (solution of the Navier-Stokes equation) and the coating formation applied to the PICVD coating of a convex and a concave substrate surface confirms this expectation.
The simulation for the production of a TiO.sub.2 coating from a TiCl.sub.4 O.sub.2 gas mixture with a convex curvature radius Ks=122 mm on the substrate surface only results in a uniformity of U=0.71 (U=relation of minimum to maximum coat thickness) for a PICVD coating with microwave stimulation and the usual coating parameters, and the application of a gas showerhead and a constant mass flow from the gas emitting surface. The uniformity is only U=0.6 with the stronger curvature Ks=80. The thickness of the coating is a maximum at the edge of the substrate and a minimum in the middle. Experiments with convex substrates demonstrate that the thickness at the edge was probably correspondingly thicker because of the higher gas column above the substrate at the edge versus a lower gas column in the middle of the substrate. A reverse result of the coating thickness was achieved with concave substrates.